3GPP Long Term Evolution, LTE, is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project, 3GPP, to improve the Universal Mobile Telecommunication System, UMTS, standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. In a typical cellular radio system, wireless devices or terminals also known as mobile stations and/or User Equipment units, UEs, communicate via a Radio Access Network, RAN, to one or more core networks. The Universal Terrestrial Radio Access Network, UTRAN, is the radio access network of a UMTS and Evolved UTRAN, E-UTRAN, is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a UE is wirelessly connected to a Radio Base Station, RBS, commonly referred to as a NodeB, NB, in UMTS, and as an evolved NodeB, eNB or eNodeB, in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE.
Carrier Aggregation
Carrier aggregation was introduced in Release 10 of the E-UTRAN standard as a means for qualifying E-UTRAN to meet the requirements for 4G (1000 Mbit/s) as well as for allowing operators with small (less than 20 MHz) scattered spectrum allocations to provide a good user experience by aggregating the scattered allocations into e.g. 10, 20 MHz or more.
In Carrier Aggregation the UE is connected to a serving cell termed Primary Cell (PCell) on what is referred to as the Primary Component Carrier (PCC). Mobility is catered for on this carrier. In case the UE is using services that require high throughput, the network may activate one or more additional serving cells, each termed Secondary Cell (SCell), on what is referred to as Secondary Component Carrier(s). The activation may happen before or after the SCell has been detected by the UE.
Two types of aggregation scenarios are considered for Release 10:                Intra-band contiguous aggregation        Inter-band aggregationand in Release 11, one more is considered:        Intra-band non-contiguous aggregation.        
For intra-band contiguous aggregation the PCell and SCell(s) are contiguous in frequency. It is required from the standard that for contiguous intra-band aggregation, the time difference between PCell and SCell is allowed to be at most ±130 ns (3GPP TS 36.104 rev 11.4.0, subclause 6.5.3). It is further assumed in the standard that for this particular scenario, one can use a single FFT to demodulate the signal from both PCell and SCell simultaneously. Thus, in practice it is required that the PCell and SCell are co-located, i.e., transmitted from the same site, since otherwise propagation delay would make it impossible to use a single FFT. For intra-band non-contiguous aggregation the timing difference is allowed to be at most ±260 ns, but no assumption is made on that the cells are co-located or that a single FFT can be used. For inter-band carrier aggregation the timing difference between the PCell and SCell is allowed to be at most ±260 ns. However for this scenario it is further assumed that the cells may be non-co-located and that the UE will have to cope with a propagation delay difference between PCell and SCell of up to ±30 μs, resulting in a maximum delay spread of ±30.26 us (3GPP TS 36.300 rev 11.5.0 Annex J).
From 3GPP LTE Rel. 12 and onwards so called inter-node radio resource aggregation is under discussion (3GPP TR 36.842). For one of the foreseen scenarios a UE may be connected to a primary cell (master cell) handled by one base station, and simultaneously connected to between one and four secondary cells (assisting cells) handled by other base station(s). In case the primary cell and secondary cell(s) are on different carriers, the UE can aggregate it similar to how it is done for the Rel. 11 deployment scenarios, with one difference namely that the cells are handled by different sites. Up to 3GPP Rel. 11 the aggregated cells were handled by the same base station with either co-located cells on different carriers but sent from the same site, or non-co-located cells on different carriers, where the carriers are using RRH (remote radio heads).
At a given location there may be multiple such layers, overlapping each other at least partially. Although current assumption in the standard is that the UE shall be capable of aggregating up to 5 carriers, there is no such limitation on the number of carriers within which the UE may be in coverage. It can be assumed that in future deployment scenarios e.g. 5G, virtually every suitable spectrum will be used in order to meet the targets for fifth generation of mobile communication systems (5G). It can also be foreseen that there will be a mix of large and small cells i.e. any combination of macro, micro, pico and femto cells, and a mix of intra-node and inter-node aggregation. Moreover for 5G mobile base stations are considered.
The macro cell is served by a wide area (WA) base station aka high power node (HPN). The maximum output power of a HPN can for example typically be between 43-49 dBm. Examples of low power nodes (LPNs) are micro node (aka medium range (MR) base station), pico node (aka local area (LA) base station), femto node (home base station (HBS)), relay node etc. The maximum output power of an LPN for example typically is between 20-38 dBm depending upon the power class. For example a pico node typically has a maximum output power of 24 dBm whereas HBS has a maximum output power of 20 dBm. The HBS, LA BS and MR BS serve femto cell, pico cell and micro cell respectively. The WA BS, HBS, LA BS and MR BS are therefore also called as different base station power classes.
A hypothetical deployment with 5 carriers is illustrated in FIG. 1a, where there are two layers with macro cells (F1 and F2), one layer with micro cells and picocells mixed (F3), one layer with picocells (F4), and one layer with femtocells (F5)—e.g. hotspots at café s, restaurants, etc. FIG. 1b illustrates cell coverage experienced by a UE that is experiencing cell coverage according to the example of FIG. 1a. The UE can be connected to one or more of the multitude of cells simultaneously, where the number of cells connected to at a given instant may depend on the throughput required for the currently used services.
Typical cell radii for the different kinds of cells are provided in Table 1 (below). The UE will go in and out of coverage of individual cells on one or more of the 5 carriers while mobile.
TABLE 1Cell types and typical cell radiusCell typeRadiusMacro>2000mMicro200-2000mPico10-200mFemto0-10mWireless Device (e.g., UE) Radio Network Related Capability Information
According to the present 3GPP standard, the wireless device reports its network capabilities, called User Equipment, UE, capabilities, in a NW registration/attach phase. 3GPP TS 36.331 V12.10.0 defines the UE capability reporting procedure and the messages sent between eNodeB and the UE.
The purpose of this procedure is to transfer UE radio access capability information from the UE to E-UTRAN. If the UE has changed its E-UTRAN radio access capabilities, the UE shall request higher layers to initiate the necessary NAS procedures (see 3GPP TS 23.401 V12.5.0) that would result in the update of UE radio access capabilities using a new RRC (Radio Resource Control) connection.
To allow for a range of user equipment (UE)/wireless device implementations, different wireless device capabilities are specified. The wireless device capabilities may be used by the network to select a configuration that is supported by the wireless device. FIG. 2b illustrates an example of UE capability transfer.
In 3GPP standardization of E-UTRAN radio access, the UE radio network related capability information is transferred using RRC (Radio Resource Control) signaling from the UE (wireless device) to the eNodeB (eNB or base station).
Information on the UE radio network related capability information has to be present in the eNB in the RRC connected state of the UE. Moreover, when a handover is made from a first/source eNB to a second/target eNB the UE capability information needs to be moved from the source eNB to the target eNB.
However, in RRC idle state there is no need to maintain any information of the UE, including the UE capabilities, in the eNBs.
In order to avoid uploading the UE capabilities over the radio interface between the UE and the eNB each time the UE performs a transition to RRC connected state (i.e. when the UE specific context is created in the eNB), the eNB uploads the UE capability information to the Mobility Management Entity (MME) in the Evolved Packet Core (EPC) so that it can be stored there when the UE is in RRC idle state. When the UE next time returns to RRC connected state the UE capability information will be downloaded from the MME to the eNB.
The UE radio network related capability information is grouped, where each group reflects a certain type of capabilities. Examples of such capability groups are:                Radio Frequency (RF) Parameters        Measurement Parameters        Inter-RAT (Radio Access Technology) Parameters        
The RF-Parameters includes, e.g., supported EUTRA frequency bands, supported EUTRA band combinations.
Measurement Parameters includes information about the UE need for DL measurement gaps while performing inter-frequency measurements when UE operating on a specific E-UTRA band or on a specific E-UTRA band combination.
Inter-RAT Parameters includes information about the supported frequency bands for each other RAT the UE support. UE Capabilities and reporting thereof is defined in 3GPP TS 36.300 (mainly section 18) and 3GPP TS 36.331.
In the following “network capability” refers to the “UE radio network related capability”.
Most network capabilities are reported in a binary manner hence a UE can only state to the network node that it is capable of the associated feature if simultaneously supporting it on all configured carriers. Moreover, a UE can only state that it is capable of a feature if it can handle it simultaneously with all other features it reports to be capable of.
With introduction of more complex methods for interference cancellation to meet denser network deployment (ultra-dense network deployment), the requirements on the baseband processing capacity on UE side increases dramatically, and therefore introduction of new features often require a new generation of baseband hardware.
In many foreseen network deployments it is still so that a few carriers are used for macro or micro cell deployment, i.e. cells with large radius providing mobility, and the rest of the carriers may be smaller cells, e.g. pico and femto-cells with small radius, depending on physical constraints on how wide area electromagnetic waves can penetrate as frequency increases. Hence, many of the features are not to be used on all configured carriers simultaneously, but only on a few of them. However, the UE has to be dimensioned to handle the feature on all carriers simultaneously in order to state capability for that feature. This implies that, in future 5 CA with five aggregated carriers, deployment scenarios it is not likely that very advanced interference cancellation will be needed on all carriers simultaneously. At least one carrier is to be a macro or micro cell to provide wide area coverage for mobility. Moreover, due to different characteristics of cells on different carriers, it is not likely that it will be meaningful using OTDOA on all configured carriers.
Further, a UE capable of aggregating say 5 carriers may have spare capacity that can be used for supporting additional features when fewer than maximum 5 carriers are configured.
Capability handling needs to become smarter in the future to avoid exploding growth of UE complexity and to allow new functionality to be introduced earlier, without having to wait for next generation of baseband hardware.